Synchronous Optical Network (SONET) is an industry standard for high-speed transmission over optical fiber. The SONET standard, as formulated by the Exchange Carriers Standards Association (ECSA) for the American National Standards Institute (ANSI), is herein incorporated by reference in its entirety. SONET defines a technology for carrying many signals of different capacities through a synchronous, flexible, optical hierarchy. Synchronous Digital Hierarchy (SDH) is a standard technology for synchronous data transmission on optical media. SDH is the international equivalent of SONET. Both technologies provide faster and less expensive network interconnection than traditional plesiochronous (almost synchronous) digital hierarchy (PDH) equipment. In digital transmission systems, synchronous means the bits from one call, for example in digital telephone transmission, are carried within one transmission frame.
Frame structures specify the distinctive, and usually cyclic, arrangement of timeslots in a transmission line for the purpose of identifying individual timeslots in a frame of information. Frame structures are also called framing formats. Framing is a method of indicating where to begin counting channels so for example, a demultiplexer knows where to start counting channels. Framing of digital signals accounts for the need to synchronize and identify data bits at four levels: bit level, byte level, frame level and system or network level. The information transmitted is composed of individual data bits. Error-free performance depends on the precise control of the bit rates throughout a network. Information is processed at the byte level (unique strings or groups of 8-bit words) by digital cross-connect and switching systems. In order for these systems to identify unique strings of bits, the serial data stream must include frame synchronization. A frame usually includes a number of data bytes.
The basic rate for SONET is 51.85 Million bits per second (Mbps) to permit more bandwidth for operation, administration and maintenance functions than the previous plesiochronous Digital Hierarchy (PDH) technology. SONET is based on the Synchronous Transport Signal level-1 or STS-1 frame which consists of 810 octets (9 rows of 90 octets). The STS-1 is a specific sequence of 810 bytes (6480 bits), which includes various overhead bytes and an envelope capacity for transporting payloads. For example, the bytes A1 and A2 are framing bytes which indicate the beginning of an STS-1 frame. The STS-1 frame is transmitted top to bottom, row by row, from left to right. The STS-1 frame is transmitted every 125 μs (8000 frames per second), thus resulting in a transmission rate of 51.84 Mbps. SONET STS-n frames are mapped onto the optical equivalent OC-n where OC stands for optical carrier. SONET data transmission rates are often given by OC-n, for example OC-1 corresponds to an optical line rate of 51.84 Mbps, and OC-48 corresponds to 2,488.32 Mbps.
SDH uses the Synchronous Transport Modules (STM) and rates for example, STM-1 operates at a speed of 155 Mbps, STM-4 operates at a speed of 622 Mbps, STM-16 operates at a speed of 2.5 giga bits per second (Gbps), and STM-64 operates at a speed of 10 Gbps. The STM-1 basic frame consists of 2430 octets (270×9) or 9 sets of timeslots with each set having 270 timeslots. Within a time slot interchange unit, a STM-1 frame is converted into a matrix of data bytes with each row being a set of timeslots and a column consisting of all occurrences of an individual timeslot in all rows. The resultant matrix has 9 rows and 270 columns. The STM-1 frame consists of a section overhead (SOH), of approximately 5.184 Mbps and a payload of 150.336 Mbps. The SOH of the STM-1 frame is reserved for SDH management, while the payload contains the information to be transmitted by customers.
For each defined rate, a portion of each data frame is dedicated to overhead with the balance being available for data payload. For example, for STS-1, the payload capacity is 49.536 Mbps.
SONET/SDH compliant fiber-optical facilities are being installed by corporations for backbone networks, as well as by carriers and competitive access providers for long-haul routes and fault-tolerant rings around major metropolitan areas. SONET/SDH combines bandwidth and multiplexing capabilities, allowing users to fully integrate voice, data, and video over a single fiber-optic facility. SONET/SDH technology standardized line rates, coding schemes, bit-rate hierarchies, operations and maintenance functionality.
As interface speeds increase, so too does the cost of optical modules. For example, at OC-192, having a line rate of 9.953 Gbps, the cost of the traditional optical transceiver modules is very high. These modules are often required to only transmit data very short distances (i.e. router to router or router-to-dense wavelength division multiplexing (DWDM) terminal within a central office) and do not need to be as highly specified as they would be if transmitting over long distances. Parallel optical transmission modules have recently become available on the market. Their main target is to address the board-to-board or rack-to-rack interconnection.